Frequency synthesizers are known to comprise a reference oscillator, a reference divider, a phase detector, an output frequency divider, a voltage control oscillator, and a multi-bandwidth filter. Typically, a frequency synthesizer provides a stable frequency output, or local oscillator, for communication equipment. The communication equipment, which may be a television, an AM radio, an FM radio, a portable communication radio, or a cellular telephone, includes a selection device such that the output frequency of the frequency synthesizer can be changed. By changing the output frequency, the communication equipment can receive a new channel. For example, when an AM radio changes its channel from 720 to 780, the output frequency of the frequency synthesizer within the radio changes from 720 kHz to 780 kHz.
As is also known, the multi-bandwidth filter, which is a low pass filter, attenuates high frequency signals and noise while allowing low frequency signals to pass. To adequately attenuate high frequency signals, the filter includes a relatively large capacitance which impedes rapid changes in output frequencies. To expedite an output frequency change, the filter is changed from a narrow bandwidth to a wide bandwidth for a fixed period of time. The change fin the bandwidth provides a lower impedance path such that the capacitance within the filter is charged, or discharged, at a faster rate. Ideally, the wide bandwidth would be as wide as possible to keep the frequency synthesizer stable, however, the bandwidth is typically a factor of 10 times the narrow bandwidth.
When the multi-bandwidth filter returns to its previous narrow bandwidth, there is a tendency of the multi-bandwidth filter to return to the previous control voltage. This phenomenon is called dielectric absorption. This causes a frequency pull-back condition to occur wherein the output frequency of the frequency synthesizer, although adjusted for the selected frequency, tends to return towards the previous frequency until the multi-bandwidth filter stabilizes, thereby increasing the frequency adjustment response time. The amplitude of the pull-back decays proportionally to the amount of time the filter is in the wide band, and increases with the amount of frequency change.
Since the duration of the frequency adjustment time is fixed, it is determined based on acceptable pull-back for the greatest expected frequency change. For example, the greatest frequency change in an AM radio is from 520 kHz to 1600 kHz. For small frequency changes, such as from 720 kHz to 780 kHz, the fixed frequency adjustment time is greater than what is needed to achieve the frequency change thereby instilling an unnecessary delay in the transition time.
Therefore, a need exists for a method of adjusting the output frequency of a frequency synthesizer that optimizes the transition time and response.